Converting a Dissimilarity Matrix

From: Adam Tee

Hi all,

I'm trying to perform a hierarchical clustering on some
dissimilarity data that I have but the data matrix I have already 
contains the dissimilarity values.  These values are calculated using
a separate program.  The dissimilarity matrix in complete with no 
missing values but the hclust, and agnes routines require it in the
form produced by daisy or dist.  Is there any of converting 
my complete matrix to this format without changing the values??

Thanks

Adam